The present invention relates to a method of controlling a nuclear power plant. More particularly, the invention concerns a controlling method which permits the output of a reactor to be controlled to a permissible maximum level while maintaining a water level within the reactor in a safe range, even when partial shutdown occurs in a feed water pump system.
In the first place, description will be made briefly of a nuclear power plant which the present invention concerns. As is well known, steam produced from a nuclear reactor is supplied to a main turbine by way of a main steam conduit and a regulating valve. Thereafter, the steam is condensed to water through a condenser. The condensate or water is then fed back to the reactor as feed water through a feed water pipe line under action of a feed water pump system.
In the nuclear power plant of this type, the feed water flow pumped by the pump system is controlled by using at least one of signals representing the water level within the reactor, the primary steam flow rate flowing through the main steam conduit and the feed water flow, respectively, so that the water level within the reactor remains constant.
The feed water pump system may be composed of four individual pumps, that is, two main pumps each of a 55%-capacity and two auxiliary pumps each of a 27.5%-capacity, where "55%" and "27.5%" indicate the ratios to the amount of the feed water flow required when the plant operates at its rating capacity. When the nuclear reactor is operated with the output in the vicinity of the rated power, the two main pumps are driven for supply feed water. The auxiliary pumps are used for backing up the main feed water pump or for starting the nuclear power plant.
With the arrangement of the feed water control system for the nuclear reactor described above, the feed water is supplied to the reactor by the two main feed water pumps of 55%-capacity in the normal operating condition. When the main feed water pump is tripped or shut down for some reasons, then the two auxiliary pumps each of 27.5%-capacity are started to assure the feed water flow to the reactor. By the way, since the main and auxiliary pumps differ from each other in respect of the capacity as mentioned above, there is necessarily brought about a difference between the primary steam flow rate and the feed water flow rate when both the main feed water pumps are tripped or shut down with the auxiliary pump failed to be started. Under such circumstance, the water level within the reactor may be lowered to an emergency level at which the reactor has to be shut down or scrammed.
However, the shutdown of the nuclear reactor (which means nothing but the shutdown of the nuclear power plant) due to abnormal condition of the feed water pumps must be avoided by all means. Once the scram occurs, an enormous time is required for restoring the plant to the normal operating condition. In this connection, it is theoretically known that the water level within the reactor is determined as a function of difference between the primary steam flow and the feed water flow. Accordingly, it will be possible to prevent the water level from being lowered to the emergency level by decreasing the primary steam flow rate, which means of course a corresponding decrease in the output power of the reactor (primary steam flow rate). Thus, it is not adequate to merely prevent the scram but desirable to be able to increase the power of the reactor as early as possible so that the nuclear power plant can be operated in a continuous manner even when the trip or partial shutdown occurs in the feed water pump system, because the reactor itself is in the normal condition.